JP4107513B1 - Method for controlling light emission of electronic device - Google Patents

Abstract

An electronic device and a light emission control method for the electronic device that can further extend the life of an electronic device including a light emitting portion that emits light by utilizing recombination of electrons and holes.
An electronic device having a light emitting portion that emits light by utilizing recombination of electrons and holes, and a drive portion that inputs a pulsed drive signal to the light emitting portion to cause the light emitting portion to emit light intermittently. The drive unit sets the duty ratio of the drive signal to a value at which light emission from the light emitting unit can be regarded as continuous light emission, and the light emission unit has the pulse width of the drive signal and the injection amount of electrons and holes into the light emitting unit Time and electrons in which both electrons and holes are trapped in the defect level and propagate or diffuse with the recombination energy released by recombination of electrons and holes at the defect level, and no recombination occurs. And the amount of holes injected.
[Selection] Figure 3

Description

The present invention relates to a light emission control method for an electronic apparatus having a light emitting portion that emits light by utilizing the recombination of electron and holes.

  In recent years, an electronic device having a light emitting portion that emits light by utilizing recombination of electrons and holes in a PN junction region is often used as a light source such as a light projecting device or a lighting device. In particular, an electronic device using a light emitting structure in which an active layer such as a quantum well active layer is provided between a P-type semiconductor layer and an N-type semiconductor layer constituting a PN junction to effectively emit light is also known. ing.

  In an electronic device including such a light emitting unit, free electrons and free holes are applied by applying a positive voltage to the P type semiconductor layer side with respect to the N type semiconductor layer side forming the PN junction of the light emitting unit. The carrier made of is injected into the active layer, and the light emission phenomenon is caused by recombining electrons and holes in the active layer. This light emission phenomenon is such that carriers are sequentially supplied to the active layer as long as a positive voltage of a predetermined magnitude is continuously applied to the P-type semiconductor layer side, so that the light emission state is maintained and light is emitted continuously. .

  In the light emitting portion, Joule heat is generated due to resistance of a semiconductor layer portion such as an N-type semiconductor layer or a P-type semiconductor layer with energization by applying a positive voltage, and the semiconductor layer is generated by the thermal energy resulting from the Joule heat. It is known that the light output of the light emitting part is reduced due to deterioration.

  Furthermore, in the semiconductor layer portion of the light emitting portion, a defect level that captures carriers composed of electrons and holes exists in advance, and electrons and holes are recombined by electron-hole recombination at this defect level. When this occurs, the recombination energy released along with this recombination is released as thermal energy instead of light energy. It has been clarified by the study by the present inventors that the proliferation or diffusion is promoted and the light output of the light emitting portion, that is, the luminance is lowered.

  Therefore, in the current injection type light emitting portion into which carriers are injected, the thermal energy of Joule heat generated when continuously driven and the electrons / electrons that do not emit light at the defect level existing in the semiconductor layer portion of the light emitting portion. Due to the fusion of thermal energy due to hole recombination, defect levels that capture carriers grow or diffuse, causing a decrease in light output that appears as a decrease in luminance, and the desired luminance cannot be obtained. It was supposed to have a lifetime.

  Therefore, conventionally, by reducing the concentration of the defect level itself generated in the semiconductor layer portion of the light emitting portion, the life of the electronic device provided with the light emitting portion is ensured by ensuring the life of the light emitting portion.

Alternatively, a pulsed drive signal is input to the light emitting unit, and the light emitting unit is turned on and off alternately to emit light intermittently, thereby suppressing heat generation in the light emitting unit and extending the life of the light emitting unit. Thus, the lifetime of the electronic device provided with the light emitting portion has been increased. In particular, in an electronic device equipped with a light emitting unit, the heat generation in the light emitting unit is suppressed by reducing the duty ratio, which is the ratio of the pulsed drive signal period and the pulse width of the drive signal, and the life of the light emitting unit (See, for example, Patent Document 1).
International Publication No. 2004/057561 Pamphlet

  However, even when the light emitting unit is driven with a pulsed drive signal, the life of the light emitting unit has not been sufficient. That is, Joule heat generated in the light emitting unit can be suppressed by reducing the duty ratio of the drive signal, but the expected temperature suppressing effect is small, and the light emitting unit is at a temperature lower than the ambient temperature. This is because there is nothing.

  In view of such a current situation, the inventors of the present invention have made the present invention by conducting research and development in order to extend the life of an electronic device including a light emitting unit by extending the life of the light emitting unit. It is a thing.

The light emission control method of the electronic device according to the present invention includes a drive unit that intermittently emits light by inputting a pulsed drive signal into the light emission unit that emits light using recombination of electrons and holes. In the light emission control method of the electronic device, the electron concentration is n, the hole concentration is p, the electron thermal velocity is v _{th: n} , the hole thermal velocity is v _{th: p} , and the defect level existing in the light emitting portion is determined. Assuming that the capture cross section for electrons is σ _n , the capture cross section for holes at the defect level existing in the light emitting section is σ _p , and the pulse width of the drive signal is W, the drive section sets the drive signal to the duty ratio Without relying on
W <1 / {n · v _{th: n} · σ _n · p · v _{th: p} · σ _p / (n · v _{th: n} · σ _n + p · v _{th: p} · σ _p )}
And the pulse width W to meet the.

Furthermore, in the light emission control method of the electronic device according to claim 1, the drive signal is:
In the case of n · v _{th: n} · σ _n << p · v _{th: p} · σ _p , W <1 / n · v _{th: n} · σ _n ,
In the case of n · v _{th: n} · σ _n >> p · v _{th: p} · σ _p , W <1 / p · v _{th: p} · σ _p ,
It also has a feature.

According to the present invention, the drive signal does not depend on the duty ratio,
W <1 / {n · v _{th: n} · σ _n · p · v _{th: p} · σ _p / (n · v _{th: n} · σ _n + p · v _{th: p} · σ _p )}
By making the pulse width W satisfying the above, the recombination of the drive signal pulse width and the injection amount of electrons and holes is released by recombination of electrons and holes at the defect level existing in the light emitting part. can be injection amount of growth or diffused defect levels into electrons and positive both holes never been captured recombination occurs pulse width and electrons and holes in energy, electrons and positive in defect levels Since recombination with holes can be suppressed, generation of recombination energy released by recombination of electrons and holes can be prevented.

  Therefore, in the light emitting portion of the electronic device, since the progress of deterioration due to the proliferation or diffusion of the defect level can be suppressed, the life of the light emitting portion can be extended, and the life of the electronic device including the light emitting portion can be extended. Can do.

  In the electronic device and the light emission control method of the electronic device of the present invention, a pulsed drive signal is input from the drive unit to the light emitting unit that emits light by utilizing recombination of electrons and holes provided in the electronic device, and the light emitting unit is installed In particular, the driving signal has a duty ratio that allows the light emission at the light emitting part to be regarded as continuous light emission, while the pulse width of the driving signal and the electrons and holes to the light emitting part. Both the electrons and holes are trapped in the defect level that is propagated or diffused by the recombination energy released by the recombination of electrons and holes in the defect level existing in the light emitting part. The time during which recombination does not occur and the amount of injection of electrons and holes suppress the recombination of electrons and holes at the defect level.

Here, the theory of electron-hole recombination at the defect level will be briefly explained. The electron-hole recombination rate U at the defect level per defect concentration N _t [cm ⁻³ ] is expressed by the following equation. It is known to be given.
U = v _{th: n} σ _n v _{th: p} σ _p (p · n-n _i ² ) /
(σ _p (p + n _i exp {(E _i -E _t ) / kT}) + σ _n (n + n _i exp {(E _t -E _i ) / kT}))
here,
U: Electron / hole recombination rate [1 / sec]
v _{th: n} : thermal velocity of electrons (= √ (3kT / m _n ^* )) [cm / sec]
v _{th: p} : Hole heat velocity (= √ (3kT / m _p ^* )) [cm / sec]
k: Boltzmann constant [J / K]
T: Absolute temperature [K]
m _n ^* : Effective mass of electrons [kg]
m _p ^* : effective mass of holes [kg]
σ _n : capture cross section for defect level electrons [cm ² ]
σ _p : Capture cross section for holes at defect level [cm ² ]
n: electron concentration [cm ^-3 ]
p: Hole concentration [cm ^-3 ]
n _i : Intrinsic carrier concentration [cm ^-3 ]
E _i : Intrinsic Fermi level [J]
E _t : Defect level [J]

The above equation is approximated by the following equation in an operating state in which carriers including electrons or holes are injected into the light emitting portion, that is, when the light emitting portion is in a predetermined energized state.
U ≒ n ・ C _n p ・ C _p / (n ・ C _n + p ・ C _p ),
here,
C _n : electron capture coefficient of defect level (C _n = v _{th: n} · σ _n ) [cm ³ / sec]
C _p : hole capture coefficient of defect level (C _p = v _{th: p} · σ _p ) [cm ³ / sec]

Usually, since the capture coefficient C _n for defect level electrons and the capture coefficient C _p for defect level holes are different, the above equation is further approximated. In this case, the electron-hole recombination rate U per defect concentration is
U ≒ n ・ C _n , (nC _n << pC _p ) [1 / sec],
U ≒ p ・ C _p , (nC _n >> pC _p ) [1 / sec],
It becomes. This means that the electron / hole recombination rate of the defect level is limited by the slower carrier capture rate.

  Further considering this equation, in order to prevent recombination of electrons and holes at the defect level in one defect, carrier injection is performed in less than the time required for recombination of electrons and holes at the defect level. Will be sufficient.

  Here, in order to prevent recombination of electrons and holes at the defect level in one defect, carriers may be injected in less than the time required for recombination of electrons and holes at the defect level. However, even if either the electron or hole is constrained to the defect level, recombination of the electron and hole does not occur unless the other hole or electron is constrained to the defect level.

  Therefore, the recombination rate is determined by the product of the electron carrier concentration and the electron carrier capture coefficient captured by the defect level, and the hole carrier concentration and the hole carrier capture coefficient captured by the defect level. Of the product values, there is no problem with the larger product value.

That is, the reciprocal of the carrier capture rate that determines the electron-hole recombination rate U, that is, 1 / n · C _n [sec] if nC _n << pC _p , nC _n >> pC _p Is less than 1 / p · C _p [sec] and carriers may be injected.

Therefore, if nC _n << pC _p , C = C _n and the carrier concentration is the electron concentration. If nC _n >> pC _p , C = C _p and the carrier concentration is the hole. As the concentration, if the pulse width of the driving signal is W [sec] and the carrier capture coefficient of the defect that determines the electron-hole recombination rate in the defect level is C [cm ⁻³ / sec], the defect level The driving condition of the light emitting unit for suppressing electron-hole recombination in the case is that the pulse width of the driving signal satisfies the condition of W <1 / nC = 1 / U [sec / 1 pulse]. In other words, the pulse width of the drive signal may be shorter than the time required for both electrons and holes to be captured at the defect level and recombination to occur.

  Note that 1 on the right side of W <1 / nC [sec / 1 pulse] is captured by one defect level during the time when the light emitting portion corresponding to the pulse width of one pulse of the drive signal is in the ON state. This means the number of carriers to be transferred, that is, the amount of carriers captured per pulse (number of pulses / pulse). Therefore, the conditional expression W gives the pulse width of the drive signal so as not to cause electron-hole recombination even once in the defect level of one defect.

  Here, when setting the cycle of the drive signal and the duty ratio of the drive signal, the cycle or the duty ratio can be freely set as long as the pulse width W of the drive signal satisfies the condition of “W <1 / U”. By increasing the value of the duty ratio as much as possible, it is possible to reduce the off-state period during which the light emitting portion does not emit light and to extend the life of the light emitting portion while apparently continuously emitting light with sufficient luminance. .

  In other words, when it is desired to increase the luminance, the drive signal cycle is reduced within a predetermined time by reducing the drive signal cycle and increasing the drive signal duty ratio as much as possible while satisfying the condition of “W <1 / nC”. Since it is possible to lengthen the time during which the is in the on state, high luminance can be achieved. Moreover, the pulse width of the drive signal satisfies the condition “W <1 / nC”, whereby the light emitting portion can have a long lifetime.

  Thus, in the present invention, the pulse width of the drive signal that drives the light emitting unit and the amount of electrons and holes injected into the light emitting unit, that is, the amount of current flowing through the light emitting unit are important. By setting the pulse width and energization amount based on the electron-hole recombination rate at the defect level inherent in the semiconductor layer part of the light emitting part, the defect propagation is suppressed and the life of the light emitting part is extended. However, it does not simply reduce the duty ratio of the drive signal to extend the life. Note that the pulse width of the drive signal is a time during which the light emitting unit is in an on state, and for convenience of explanation, it will be referred to as an “element drive time” in the following.

  Note that in the semiconductor layer portion of the light emitting portion, not only a single defect level but also a plurality of defect levels are often present.

  Therefore, the device drive time (pulse width of the drive signal) and the current value of the drive signal that defines the amount of electrons and holes injected into the light emitting part are the device drive time and current value that match the defect level to be suppressed. If set, it is possible to suppress element deterioration due to a defect causing the defect level.

That is, for example, when defect type 1 and defect type 2 having different electron / hole recombination rates exist, electron / hole recombination rate n ₁ C _{1 of} defect type 1 and electron / hole recombination rate of defect type 2 Of the hole recombination rate n ₂ C ₂ , by setting the element drive time and the current value of the drive signal based on the electron / hole recombination rate of the defect type that you want to suppress, the element degradation due to the defect type Can be suppressed.

  Alternatively, if a higher electron / hole recombination rate is selected, not only the defect type corresponding to the electron / hole recombination rate, but also a lower value of electron / hole recombination rate. Deterioration of the element due to the defect type corresponding to the above can be suppressed, and the lifetime of the light emitting element can be extended most by selecting the electron / hole recombination rate having the largest value.

  Further, in a light emission control circuit that performs a light emission control of a light emitting element by outputting a required drive signal to the light emitting element, an output time of the output drive signal, an element drive time and a drive signal in the output drive signal From the duty ratio, a cumulative time obtained by sequentially accumulating the light emission time during which the light emitting element emits light may be stored, and the pulse width of the drive signal may be changed according to the cumulative time of the light emission time.

  That is, for example, while the value of the cumulative time of the light emission time is small, the element driving time is set relatively long due to the small number of defects, and the value of the cumulative time of the light emission time becomes large, which causes natural deterioration, etc. As the number of defects increases due to the influence of the above, the element driving time may be gradually shortened.

  Hereinafter, embodiments of the present invention will be specifically described with reference to the drawings. As shown in FIG. 1, an electronic device 10 of this embodiment includes a light emitting unit 20 ′ having a light emitting element that emits light by utilizing recombination of electrons and holes, and a pulse-like drive for the light emitting unit 20 ′. And a driving unit 30 for inputting a signal and causing the light emitting unit 20 ′ to emit light intermittently.

  Here, the electronic device 10 may be anything, such as a lighting device or a display device provided with a light emitting element, and specifically, the lighting device was adjusted to irradiate light of a predetermined wavelength. Examples include lighting fixtures, headlights such as automobiles, motorcycles, and bicycles, searchlights, flashlights, penlights, and backlights for liquid crystal display devices. The display device is a device constituted by one or a plurality of light emitting diodes such as a traffic light and a warning light.

  In the following, the light emitting element of the light emitting unit 20 ′ includes a zinc selenide (ZnSe) -based white LED (Light Emitting Diode), which is a II-VI group compound semiconductor, and gallium nitride (GaN), which is a III-V group compound semiconductor. Although the example implemented by the system ultraviolet LED is shown, the light-emitting element is not limited to the light-emitting element composed of a crystal material, but an active layer such as a quantum well layer sandwiched between a P-type semiconductor layer and an N-type semiconductor layer What is necessary is just a so-called carrier injection type light-emitting element provided with

A ZnSe white LED 20 which is a light emitting element of the present embodiment is formed using an N-type semiconductor layer 21 formed using zinc chloride (ZnCl ₂ ) as an n-type dopant and nitrogen (N ₂ ) gas as a p-type dopant. The P-type semiconductor layer 22 is configured as a PIN diode with a ZnCdSe / ZnSe multiple quantum well active layer 23 sandwiched therebetween.

  The N-type semiconductor layer 21 and the P-type semiconductor layer 22 are connected to the light emission control circuit of the drive unit 30 through electrodes, respectively, and a predetermined drive pulse current is supplied as a drive signal to the ZnSe white LED 20 by this light emission control circuit. The ZnSe white LED 20 is intermittently emitted. In the light emission control circuit, the element drive time in the drive pulse current is appropriately adjusted and output.

  Specifically, the ZnSe-based white LED 20 of this embodiment includes a conductive n-type ZnSe single crystal (100) as a substrate, and a titanium (Ti) film and a gold (Au) film on the lower surface of the substrate. Are stacked to form an electrode.

  On the single crystal ZnSe substrate, the following semiconductor layers are formed by a molecular beam epitaxy (MBE) method. In forming each semiconductor layer, zinc (Zn), magnesium (Mg), cadmium (Cd), sulfur (S), selenium (Se), and tellurium (Te) having a purity of 6 nines are appropriately supplied by a Knudsen cell. The epitaxial thin film crystal is grown.

On the single crystal ZnSe substrate, an n-ZnSe buffer layer of about 1.0 μm and an n-ZnMgSSe cladding layer of about 0.5 μm are formed while adding zinc chloride (ZnCl ₂ ) as an n-type dopant. An N-type semiconductor layer 21 is formed together with the crystal ZnSe substrate. The effective carrier concentration in the n-ZnSe buffer layer was 7 × 10 ¹⁷ cm ⁻³ , and the effective carrier concentration in the n-ZnMgSSe cladding layer was 5 × 10 ¹⁷ cm ⁻³ .

  On the top surface of the n-ZnMgSSe cladding layer, an approximately 0.03 μm i-ZnSe carrier confinement layer, an approximately 0.01 μm ZnCdSe / ZnSe multiple quantum well active layer 23, and an approximately 0.03 μm i-ZnSe layer are sequentially formed. A p-type semiconductor layer 22 is formed on the i-ZnSe layer.

The P-type semiconductor layer 22 includes a p-ZnMgSSe layer of about 0.5 μm, a p-ZnSe layer of about 0.5 μm, a multiple quantum well ZnSe / ZnTe layer of about 40 nm, and a p-ZnTe contact layer of about 40 nm. The metal electrodes are formed by depositing gold (Au) on the upper surface of the p-ZnTe contact layer. The effective carrier concentration in the p-ZnMgSSe layer is 3 × 10 ¹⁶ cm ⁻³ , the effective carrier concentration in the p-ZnSe layer is 4 × 10 ¹⁷ cm ⁻³ , and the effective carrier concentration in the p-ZnTe contact layer is 2 × 10 ¹⁹ cm ^{−. It was set to 3} .

  Here, the multiple quantum well ZnSe / ZnTe layer is called a superlattice electrode, and is provided to form a pseudo-ohmic electrode layer on a p-type ZnSe crystal. By providing the quantum well ZnSe / ZnTe layer and the p-ZnTe contact layer, holes can be transported between the p-ZnSe layer and the p-ZnTe contact layer by the resonant tunneling effect.

The thus configured ZnSe white LED 20 (hereinafter simply referred to as “LED element”) was fixed to a cryostat sample holder, the inside of the cryostat was set to 10 ⁻⁴ Pa or less, and the LED element was driven with a driving pulse current.

FIG. 2 shows device drive using the temperature of the sample holder in the cryostat as 333 K, the density of the drive pulse current injected into the LED device 20 as 20 A / cm ² , and the device drive time measured under accelerated deterioration test conditions as parameters. It is the result of an experiment.

  In this element drive experiment, the drive pulse current cycle is set to 10 msec, the element drive time of each drive pulse current (pulse width of the drive pulse current) is measured as 7.5 msec, 5 msec, and 1 msec, respectively. Measurement was performed when 20 was always energized and operated continuously. In FIG. 2, the time in the drive pulse current is the cumulative time of the element drive time portion in the drive pulse current.

  When the LED element 20 is continuously operated, the half-life is about 3 hours. When the element driving time is 5 msec, the half-life is about 80 hours, which is improved by about 25 times. I understand that In particular, when the element driving time is 5 msec, the actual element driving time is about 160 hours.

FIG. 3 is a graph showing the element driving time dependence of the half-life of the LED element 20. The half-life on the vertical axis of the graph is a value converted to continuous operation. From this graph, it is clear that the lifetime of the LED element 20 depends on the element driving time (pulse width of the driving pulse current) in the driving pulse current. In particular, when the element driving time is shorter than 1 × 10 ⁻² sec, the half-life of the LED element 20 exceeds 100 hours, reaching about 50 times the half-life of continuous operation. .

  It will be explained that the fact that the element driving time is shortened in this way is due to satisfying the above-described conditional expression: W <1 / nC (sec / 1 pulse).

  First, the cause of element deterioration when the LED element 20 is continuously operated will be described. This element deterioration is caused by trapping of free holes in a composite defect called H0 defect caused by nitrogen added as an acceptor to the P-type semiconductor layer 22, and then from the active layer 23 to the P-type semiconductor during the operation of the LED element 20. By capturing the overflowed electrons in the layer 22, recombination due to non-emission of electrons and holes occurs at the H0 defect level, and the H0 defects diffuse into the active layer due to the energy released by this recombination. It is known that it has occurred. That is, carrier capture, which is simultaneous capture of electrons and holes by H0 defects, is a driving force for device deterioration.

The carrier capture cross section σ in this H0 defect is 10 ^-22 [cm ² ] for free holes, measured by Double Carrier Deep Level Transient Spectroscopy (DC-DLTS). Is known to be 10 ⁻¹⁸ [cm ² ], and during the operation of the LED element 20, the electron capture rate nC _n = 10 ⁸ (1 / sec. ), The hole capture rate pC _p = 10 ² (1 / sec), and nC _n >> pC _p , so the electron-hole recombination ratio U per defect concentration at the H0 defect level is U≈pC It is given by _p = 10 ² (1 / sec) and is limited by hole trapping.

Therefore, in order to suppress the recombination of electrons and holes at this H0 defect level, the element driving condition is set to W <1 / pC _p and the element driving time W is set to W <10 ⁻² sec. That's fine.

It can be seen that this condition (W <10 ⁻² sec) is in good agreement with the experimental values shown in FIG. That is, when the LED element 20 is driven under the condition of W <10 ⁻² sec, the recombination of electrons and holes can be suppressed by suppressing the capture of holes or electrons in the H0 defect level, and the H0 defect Proliferation or diffusion is suppressed, and element deterioration can be suppressed and the life can be extended.

In the region where the element drive time in the graph of FIG. 3 is 1 × 10 ⁻³ sec <W <1 × 10 ⁻⁶ sec, the element lifetime is saturated in about 100 hours, whereas W> In the region of 1 × 10 ⁻⁶ sec, the device life is further extended. This indicates that a defect different from the H0 defect exists.

  As a defect different from the H0 defect, a compensation-type donor defect that grows in the P-type semiconductor layer 22 is known. In particular, it is known that this donor defect also propagates or diffuses with energy generated by recombination due to non-luminescence of electrons and holes at the defect level.

It is known that the carrier capture cross section σ in the donor defect is 10 ⁻¹⁷ [cm ² ] with respect to free holes as measured by transient capacitance spectroscopy, and the hole heat velocity v _th is 2 Since × 10 ⁷ [cm / sec], the element driving time W <10 ⁻⁷ sec is required from the conditional expression of W <1 / nC.

Therefore, when the element driving time W is less than 10 ⁻⁷ sec, it is possible to suppress the capture of holes or electrons at the defect level in the donor defect, so that the recombination of electrons and holes can be suppressed, and the donor property is reduced. It can be seen that the growth or diffusion of defects is suppressed, the device deterioration can be suppressed and the lifetime can be extended, and the experimental values shown in the graph of FIG. 3 are in good agreement.

  Thus, as shown in FIG. 3, it is apparent that the LED element 20 can have a longer lifetime as the element driving time W is shortened.

  In particular, when there are defect levels due to different defect types in the LED element 20, the defect type that causes the trapping of carriers in the shortest time, that is, the defect type with the highest electron-hole recombination rate nC. On the other hand, the longest element lifetime can be obtained by determining the element driving time W from W <1 / nC.

  In addition, when the defect level resulting from a different defect type exists in the LED element 20, the defect of the defect type to be suppressed is appropriately selected by appropriately selecting the element driving time W according to the defect level of each defect type. Electron / hole recombination at the level can be effectively suppressed, and the growth or diffusion of defects of the defect species can be effectively suppressed.

  The result of the device lifetime measurement experiment shown in FIG. 3 can be used in reverse to determine the electron-hole recombination rate at the defect level that controls the device degradation. That is, in the graph of FIG. 3, the reciprocal of the element driving time when the element lifetime is greatly increased is the electron / hole recombination rate, and the electron / hole recombination rate in the H0 defect, the donor defect, etc. It is a value.

  The effect of extending the element lifetime by shortening the element driving time W shown in the graph of FIG. 3 is that, as described above, electrons and holes are captured at the defect level by the shortening of the element driving time W and recombination is performed. This is because the duty ratio of the driving pulse current is reduced by shortening the element driving time W, thereby suppressing Joule heat generated in the LED element and increasing the temperature of the LED element. It is conceivable that the life is prolonged and the life is extended.

  Therefore, the relationship between the duty ratio of the drive pulse current and the temperature of the LED element was verified. FIG. 4 is a graph based on the measurement result of the temperature of the active layer when the predetermined duty ratio is set. Here, the temperature of the active layer was estimated from a basic experiment, and was estimated by comparing the peak shift characteristic depending on the temperature of the emission spectrum with the duty ratio dependency of the peak shift of the emission spectrum.

As shown in FIG. 4, it was confirmed that when the duty ratio was reduced, the temperature of the active layer was decreased, and in particular, decreased from about 344K to about 334K. In this experiment, the LED element 20 was accommodated in a cryostat having a temperature of 333 K and a pressure of 10 ⁻⁴ Pa or less. The driving pulse current applied to the LED element 20 was a rectangular wave with a period fixed at 20 msec and an element driving time of 1 to 10 msec. The current density of the drive pulse current was 20 A / cm ² , the offset voltage was about −10 V, and the applied voltage during the element drive time was about 2.5 V.

  In the experiment in which the graph of FIG. 3 was obtained, the duty ratio was 50%, and it is estimated from the graph shown in FIG. 4 that the temperature decrease is about 5 ° C.

  If it is assumed that the voltage has decreased from 344K to 334K, the device lifetime at 343K is about 2 hours according to the 1 / T characteristic graph of the half-life of the LED device 20 in the device degradation caused by the H0 defect shown in FIG. Since the element lifetime at 334 K is about 10 hours, the effect of extending the lifetime of the LED element due to a pure temperature drop can be estimated to be about five times. Therefore, as shown in FIG. 3, the effect of about 50 times is not obtained, and it is obvious that the effect of extending the device life is due to the shortening of the device driving time. Here, 1000/344 [K] ≈2.9 [1 / K] and 1000/334 [K] ≈3.0 [1 / K].

  Similarly, according to the 1 / T characteristic graph of the half-life of the LED element in the element deterioration caused by the donor defect shown in FIG. 6, the element life at 344K is about 30 hours and the element life at 334K is about 60 hours. Therefore, it can be estimated that the effect of extending the lifetime of the LED element due to a pure temperature drop is about twice. Therefore, as shown in FIG. 3, the effect of about 13 times is not obtained, and it is obvious that the effect of extending the device life is due to the shortening of the device driving time.

  Further, if the heat generation is suppressed by reducing the duty ratio and the life of the LED element is extended, as shown in the graph of FIG. 3 by shortening the element driving time without changing the duty ratio. It cannot be explained that there is an extension effect with two steps in the device lifetime.

  Therefore, it is clear that the effect of extending the measured lifetime of the LED element is not the effect due to the temperature decrease of the LED element due to the reduced duty ratio.

FIG. 7 is a graph based on the result of measuring the half life of the LED element 20 by changing the duty ratio with the element drive time of the drive pulse current input to the LED element 20 being a constant value of 5 msec. Here, the temperature in the cryostat was set to 333 K, and the current density of the drive pulse current for energizing the LED element 20 was set to 20 A / cm ² . The offset voltage of the drive pulse current was 0V.

  As shown in FIG. 7, it is clear that the element life of the LED element 20 does not depend on the duty ratio, and in order to extend the life of the LED element 20, the element drive time (pulse width) of the drive pulse current is controlled. It turns out that only is important.

  In the following, the case of a gallium nitride (GaN) ultraviolet LED that is a III-V group compound semiconductor will be described.

A GaN-based ultraviolet LED element includes an N-type semiconductor layer formed using silicon (Si) with monosilane (SiH ₄ ) as an n-type dopant and a methylcyclopentadienyl-magnesium ((C It is configured as a PIN diode with an InGaN / GaN multiple quantum well active layer sandwiched between a P-type semiconductor layer formed using magnesium (Mg) with ₅ H ₅ ) ₂ Mg) as a supply source.

Specifically, the GaN-based ultraviolet LED 20 is configured using a single crystal sapphire substrate (0001), and on this substrate, a metal organic vapor phase epitaxy (MOVPE) method is used. The semiconductor layer is formed. For the growth of each semiconductor layer, liquid trimethyl gallium (Ga (CH ₃ ) ₃ ) for supplying gallium, ammonia (NH 3) for supplying nitrogen, and aluminum are supplied using hydrogen as a carrier gas. Therefore, trimethylaluminum (Al (CH ₃ ) ₃ ) and solid trimethylindium (In (CH ₃ ) ₃ ) are appropriately supplied to grow an epitaxial thin film crystal on a single crystal sapphire substrate.

On the single crystal sapphire substrate, an n-type semiconductor layer is formed by forming an n-GaN buffer layer of about 5.0 μm and an n-AlGaN cladding layer of about 0.5 μm while adding silicon (Si) as an n-type dopant. Is configured. The effective carrier concentration in the n-GaN buffer layer is 2 × 10 ¹⁸ cm ⁻³ , and the effective carrier concentration in the n-AlGaN cladding layer is 5 × 10 ¹⁷ cm ⁻³ .

  On the top surface of the n-AlGaN cladding layer, an approximately 0.03 μm i-GaN carrier confinement layer, an approximately 0.01 μm InGaN / GaN multiple quantum well active layer 23, and an approximately 0.03 μm i-GaN layer are sequentially formed. A P-type semiconductor layer is formed on the i-GaN layer.

The P-type semiconductor layer is formed by sequentially stacking a p-AlGaN layer of about 0.1 μm, a p-AlGaN / GaN superlattice cladding layer of about 0.5 μm, and a p-GaN contact layer of about 0.1 μm. In addition, nickel (Ni) and gold (Au) are vapor-deposited on the upper surface of the p-GaN contact layer to form a metal electrode. The effective carrier concentration in the p-AlGaN layer is 5 × 10 ¹⁷ cm ⁻³ , the effective carrier concentration in the p-AlGaN / GaN superlattice cladding layer is 2 × 10 ¹⁸ cm ⁻³ , and the effective carrier concentration in the p-GaN contact layer is 1 × 10 ¹⁹ cm ^-3 .

  Here, in order to form a metal electrode on an N-type semiconductor, a required mask is formed on the single crystal sapphire substrate using photolithography technology, and the single crystal sapphire substrate is etched until the n-GaN buffer layer is exposed. Then, titanium (Ti) and gold (Au) were deposited in the opening formed by this etching to form a metal electrode as an ohmic electrode.

The GaN-based UV LED configured as described above was fixed to a cryostat sample holder, the inside of the cryostat was set to 10 ⁻⁴ Pa or less, and the GaN-based UV LED was driven with a driving pulse current having a predetermined element driving time. FIG. 8 shows the light of the GaN-based UV LED under accelerated deterioration test conditions in which the temperature of the sample holder in the cryostat is 450 K, the drive pulse current to be applied to the GaN-based UV LED is 75 mA, and the current density is 83 A / cm ^2. It is an output transition graph. The drive pulse current was a rectangular wave, the element drive time (pulse width) was 50 nsec, and the duty ratio was 25%. The element drive time in FIG. 8 is a value converted into a continuous operation time.

  From FIG. 8, the time required for the light output of the GaN-based ultraviolet LED to decrease to 80% is 1.7 hours in the case of continuous operation, whereas the above-mentioned driving which is a pulsed current. When driven by a pulse current, it is 43 hours, which is about 25 times.

  As described above, it is obvious that the life extension of the light emitting element by shortening the element driving time in the driving pulse signal for driving the light emitting element is not limited by the crystal material constituting the light emitting element.

  Therefore, ZnCdO-based, ZnMgO-based, ZnBeO-based, ZnOTe-based II-VI group zinc oxide (ZnO) -based light-emitting devices, III-V group gallium arsenide (GaAs) -based light-emitting devices, aluminum / gallium / arsenic (AlGaAs) ) -Based light-emitting devices, gallium-phosphorus (GaP) -based light-emitting devices, indium-phosphorus (InP) -based light-emitting devices, aluminum nitride-based (AlN) light-emitting devices, boron nitride (BN) -based light-emitting devices, InAs-based devices Light emitting element, GaAsP light emitting element, InGaAsP light emitting element, InGaP light emitting element, InN light emitting element, InGaN light emitting element, AlGaN light emitting element, InAlGaN light emitting element, GaInNAs light emitting element The same effect can be expected for an element and the like, and it is possible to extend the life of a carrier injection type semiconductor light emitting element utilizing recombination of electrons and holes, an LED, a laser diode, and the like.

  In particular, a light-emitting element that emits light by utilizing recombination of electrons and holes is not limited to a light-emitting element formed of a semiconductor material, and may be an organic electroluminescence, an inorganic electronic material, a phosphor, or the like. Including an optical output device in which the optical output decreases due to degradation caused by recombination of electrons and holes at the defect level present in the material. The effect of extending the lifetime can be expected by shortening the element driving time and adjusting the current value in the driving pulse signal.

  That is, for example, it is known that the decrease in luminance that occurs when an organic electroluminescent element is driven, that is, the deterioration of the element proceeds by the growth of defects with oxygen or water in the thin film serving as a nucleus. However, this defect growth also proceeds by the recombination energy released by recombination of electrons and holes at the defect level, so that the device drive time in the drive pulse signal is shortened, and the drive pulse The life extension effect can be expected by adjusting the current value in the signal.

Similarly, phosphors using electron-hole recombination, such as yttrium, aluminum, garnet (YAG), silver / aluminum-doped zinc sulfide (ZnS: Ag, Al), copper / aluminum-doped zinc sulfide (ZnS: Cu) , Al), europium-doped yttrium oxygen (Y ₂ O ₂ S: Eu), zinc and silicon oxides (Zn ₂ SiO ₄ ), calcium nitride / aluminum / silicon (CaAlSiN ₃ ), etc. Is caused by the growth of defects in the phosphor, and this defect growth also proceeds by the recombination energy released by the recombination of electrons and holes in the defect level. The effect of extending the life can be expected by shortening the time and adjusting the current value in the drive pulse signal.

  In the above-described embodiment, the driving pulse current is a rectangular wave, but is not limited to a rectangular wave, and may be a triangular wave, a sine wave, a waveform of a predetermined shape, or the like, and is not necessarily a periodic pulse. There is no need to be a wave, and the element driving time W only needs to be less than the reciprocal value of the electron / hole recombination rate nC, that is, W <1 / nC.

  In addition, each element driving time of the driving pulse current does not always need to be shorter than “1 / nC”, and an element driving time longer than “1 / nC” in order to satisfy a required luminance condition. The element drive time may be shorter than “1 / nC” at predetermined intervals.

  Adjustment of the period and duty ratio of the drive pulse current is performed by the light emission control circuit of the drive unit, and the drive pulse current may be input to the light emission unit while switching the waveform as necessary.

  In particular, in the light-emitting element provided in the light-emitting portion, not only the above-described defect multiplication or diffusion due to electron-hole recombination at the defect level occurs, but also defects may naturally occur due to cosmic rays, etc. Therefore, the light emission control circuit of the drive unit measures the total operating time of the light emitting element, and supplies the light emitting element with a drive pulse current that adjusts the element driving time based on this total operating time. May be.

1 is a schematic diagram of an electronic device according to an embodiment of the present invention. It is a graph which shows the element drive time characteristic of the light output of a ZnSe type | system | group white LED element. It is a graph which shows the element drive time characteristic dependence of the half life of a ZnSe type | system | group white LED element. It is a graph which shows the duty ratio dependence of the temperature in the active layer of a ZnSe type | system | group white LED element. It is a graph which shows the 1 / T characteristic of the half life of the ZnSe type | system | group white LED element in the element deterioration resulting from a H0 defect. It is a graph which shows the 1 / T characteristic of the half life of the ZnSe type | system | group white LED element in the element deterioration resulting from a donor property. It is a graph which shows the duty ratio dependence of the half life of a ZnSe type | system | group white LED element. 3 is a graph showing device driving time characteristics of light output of a GaN-based ultraviolet LED device.

Explanation of symbols

10 Electronic equipment
20 'Light emitting part
20 ZnSe white LED
21 N-type semiconductor layer
22 P-type semiconductor layer
23 ZnCdSe / ZnSe multiple quantum well active layer
30 Drive unit

Claims (2)

In a light emission control method of an electronic device having a driving unit that intermittently emits light by inputting a pulsed driving signal into a light emitting unit that emits light using recombination of electrons and holes,
The electron density is n, the hole density is p, the electron thermal velocity is v _{th: n} , the hole thermal velocity is v _{th: p} , and the capture cross section for electrons at the defect level existing in the light emitting part is σ _n , Σ _{p is the} capture cross-sectional area for holes in the defect level existing in the light emitting part, W is the pulse width of the drive signal,
The drive unit, without depending on the duty ratio, the drive signal,
W <1 / {n · v _{th: n} · σ _n · p · v _{th: p} · σ _p / (n · v _{th: n} · σ _n + p · v _{th: p} · σ _p )}
A light emission control method for an electronic device , wherein the pulse width W satisfies the following. The drive signal is
In the case of n · v _{th: n} · σ _n << p · v _{th: p} · σ _p , W <1 / n · v _{th: n} · σ _n
2. The case of n · v _{th: n} · σ _n >> p · v _{th: p} · σ _p , wherein W <1 / p · v _{th: p} · σ _p . A light emission control method for an electronic device.

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